Membrane electrode assembly, electrochemical cell, and electrochemical device

ABSTRACT

A membrane electrode assembly includes a pair of electrodes, each having a feeder layer that is porous and made of a conductive material, and an electrolyte membrane disposed between the pair of electrodes. At least one of the electrodes has a catalyst layer disposed in the feeder layer. In a cross section of the feeder layer, an electrolyte exists in a first region less than or equal to 80% of a thickness of the feeder layer from the electrolyte membrane toward an opposite direction to the electrolyte membrane, the catalyst layer exists at 50% or more of an outer circumference of a cross section of the conductive material in the first region, and the catalyst layer exists at 10% or less of the outer circumference of the cross section of the conductive material in a second region other than the first region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2017-172248, filed on Sep. 7, 2017, andthe entire contents of which are incorporated herein by reference.

FIELD

Embodiments relate to a membrane electrode assembly, an electrochemicalcell, and an electrochemical device.

BACKGROUND

In recent years, studies on electrochemical cells have been activelyconducted.

Among electrochemical cells, for example, a hydrogen production deviceincludes a system that causes an oxidation reaction at an anode toelectrolyze water and causes a reduction reaction at a cathode togenerate hydrogen gas.

In addition, among the electrochemical cells, for example, a fuel cellincludes a system that has an electrochemical reaction of fuel such ashydrogen with an oxidizing agent such as oxygen to generate electricity.

Among the fuel cells, a polymer electrolyte membrane fuel cell (PEFC)has been put into practical use as a household stationary power sourceand a car power source because of low environmental load.

In the electrochemical cell, a feeder and an electrolyte membraneincluded in the electrode of the electrochemical cell are peeled off dueto the gas and the like generated by the electrochemical reaction.Therefore, the feeder is deeply inserted into the electrolyte membraneto prevent the feeder and the electrolyte membrane from being peeledoff.

However, in the case of an electrochemical cell using an alternatingcatalyst layer structure (ACLS) catalyst having a unique laminatedstructure including an aggregate layer and a void layer which thepresent inventors are studying, it can be appreciated that anovervoltage of the electrochemical cell is increased if the feeder istoo deeply inserted into the electrolyte membrane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic configuration diagram of a membrane electrodeassembly according to an embodiment.

FIG. 2 is an enlarged cross-sectional view of a feeder according to anembodiment.

FIG. 3 is a schematic configuration diagram of an electrochemical cellaccording to an embodiment.

FIG. 4 is a schematic configuration diagram of an electrochemicaldevice.

DETAILED DESCRIPTION

Hereinafter, embodiments will be described with reference to thedrawings. Like reference numerals designate like components. Further,drawings are schematically or conceptually shown, the relationshipbetween thicknesses and widths of the respective portions, the ratiocoefficient of dimensions between the parts and the like are notnecessarily the same as the actual ones. In addition, even in the caseof representing the same portion, the dimensions or ratio coefficientsof the parts may be differently shown on the drawings.

FIG. 1 shows a membrane electrode assembly (Hereinafter, referred to as“MEA”) 10.

The MEA 10 includes a pair of electrodes 11 and 12 disposed facing eachother and an electrolyte membrane 13 disposed between the electrodes 11and 12.

The MEA 10 is used, for example, in an electrochemical cell of ahydrogen production device. The hydrogen production device is a devicefor producing hydrogen gas by an electrolysis reaction of water usingthe MEA 10. In the hydrogen production device, water is supplied to ananode of the MEA 10 and oxygen is generated at the anode according tothe electrolysis reaction of water. In addition, hydrogen is generatedat a cathode of the MEA 10 according to the electrolysis reaction ofwater at the anode.

The MEA 10 will be described taking the electrolysis reaction of waterof the hydrogen production device as an example.

<Electrode>

The pair of electrodes 11 and 12 is disposed facing each other. Theelectrode 11 includes a feeder layer 16 (power supply body) and acatalyst layer 14. The catalyst layer 14 is located between the feederlayer 16 and the electrolyte membrane 13. The electrode 12 includes afeeder layer 17 and a catalyst layer 15. The catalyst layer 15 islocated between the feeder layer 17 and the electrolyte membrane 13.

If the MEA 10 is used for the electrolysis reaction of water, forexample, the electrode 11 is provided as a cathode and the electrode 12is provided as an anode. The anode decomposes water by an oxidationreaction to generate hydrogen ions and oxygen. The cathode generateshydrogen by a reduction reaction. Hereinafter, the electrode 11 will bedescribed as the cathode and the electrode 12 will be described as theanode.

<Electrolyte Membrane>

The electrolyte membrane 13 is disposed between the pair of electrodes11 and 12.

The electrolyte membrane 13 contains, for example, an electrolyte havingcation exchange property. The electrolyte membrane 13 serves to conducthydrogen ions, which are generated at the electrode 12 and derived fromwater, to the electrode 11. As the electrolyte having the cationexchange property, for example, a fluororesin (for example, Nafion(registered trademark) (manufactured by DuPont Co.) having a sulfonategroup, Flemion (registered trademark) (manufactured by Asahi KaseiCorporation), and Ashiburekku (registered trademark) (manufactured byAsahi Glass Co., Ltd)) or a hydrocarbon-based film and the like, orinorganic matters such as tungstic acid or phosphotungstic acid can beused.

In addition, the electrolyte membrane 13 includes, for example, anelectrolyte having anion exchange property. Examples of the electrolytehaving the anion exchange property may include A201 (manufactured byTokuyama Corporation) and the like.

A thickness of the electrolyte membrane 13 is appropriately determinedin consideration of characteristics of the MEA 10. From the viewpointsof strength, solubility resistance, and output characteristics of theMEA 10, the thickness of the electrolyte membrane 13 is preferably 5 μmor more and 300 μm or less, more preferably, 5 μm or more and 200 μm orless.

<Feeder Layer>

The feeder layers 16 and 17 are made of a conductive material. Thefeeder layers 16 and 17 are porous. The feeder layers 16 and 17 arerequired to have sufficient gas diffusibility and conductivity.

As the conductive material of the feeder layer 17 contained in theelectrode 12 as the anode, titanium (Ti) can be used for securingdurability. As the Ti of the feeder layer 17, mesh produced by expandedmetal or etching, metal non-woven fabric in which metal fibers areintertwined, foamed metal, sintered metal and the like can be used.

Examples of other conductive materials used for the feeder layer 17 mayinclude metal elements such as tantalum (Ta), nickel (Ni), and platinum(Pt), alloys thereof, or stainless steel (SUS). These conductivematerials may be selectively used depending on a reaction potential ofthe anode in an electrochemical cell to be described later. In addition,the conductive material of the feeder layer 17 can be confirmed based ona pH-potential diagram and the like. For example, in the case of thefeeder of the anode used for the production of sodium hydroxide, Ni orSUS are eluted and therefore cannot be used. Therefore, it is preferableto use Ti as the conductive material of the feeder layer 17.

Examples of the feeder layer 16 included in the electrode 11 as thecathode may include metal materials such as Ta, Ti, SUS, Ni, andplatinum (Pt) or carbon paper, carbon cloth, metal felt, and metalnon-woven fabric and the like in which metal fiber are intertwined.

The feeder layers 16 and 17 are porous, such that gas or liquid passesthrough. When considering the movement of the material, the porosity ofthe feeder layers 16, 17 may preferably be 20% or more and 95% or less,more preferably 40% or more and 90% or less. For example, when thefeeder layers 16 and 17 are the metal non-woven fabric in which themetal fibers are intertwined, a fiber diameter is preferably 1 μm ormore and 500 μm or less and when considering reactivity and feedingproperty, the fiber diameter is more preferably 1 μm or more and 100 μmor less. When the feeder layers 16 and 17 are a particle sintered body,a particle diameter is preferably 1 μm or more and 500 μm or less andwhen considering the reactivity and the feeding property, the particlediameter is preferably 1 μm or more and 100 μm or less.

In the case of PEFC, the feeder layers 16 and 17 preferably contain awater repellent agent. For example, the water repellent agent enhancesthe water repellency of the feeder layers 16 and 17 and prevents aflooding phenomenon from occurring. Examples of the water repellentagent may include fluorine-based polymer materials such aspolytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF),polyhexafluoropropylene, tetrafluoroethylene-hexafluoropropylenecopolymer (FEP) and the like. In the case of the PEFC, generally, waterrepellency (weight percentage of a water repellent agent in a gasdiffusion layer) is preferably 2 wt % or more and 30 wt % or less. Itshould be noted that it is possible to improve properties of the PEFC byproviding a microporous layer between the feeder layers 16 and 17 andthe catalyst layers 14 and 15, and a thickness of the microporous layeris preferably 1 μm or more and 50 μm or less, more preferably 2 μm ormore and 30 μm or less. In addition, it is possible to improve thecharacteristics or the robustness of the PEFC by adjusting the porosityand the water repellency or the hydrophilic property (weight percentageof the water repellent agent or the hydrophilic agent in the microporouslayer) of the microporous layer. The hydrophilic agent may preferably bea hydrophilic material. Examples of the hydrophilic agent may includeorganic matters having a hydrophilic group or ceramic materials such asSiO₂, Al₂O₃, TiO₂ and the like.

<Catalyst Layer>

The catalyst layers 14 and 15 exist on the surface of the conductivematerial of the feeder layers 16 and 17.

The catalyst layers 14 and 15 contain a catalyst material. The catalystmaterial of the catalyst layers 14 and 15 are selectively used accordingto the reaction of the electrodes 11 and 12.

As the catalyst material of the catalyst layer 15 of the electrode 12 asthe anode, iridium oxide is mostly used because of its excellent waterelectrolysis performance and durability. In addition, precious metalcatalysts such as Pt and palladium (Pd) or lead oxide, ruthenium oxideand iridium composite oxide, ruthenium composite oxide, other oxidecatalysts and the like may be used in the catalyst layer 15. Examples ofcomposite metals forming oxides such as the iridium composite oxide andthe ruthenium composite oxide may include at least any one kind selectedfrom the group consisting of Ti, niobium (Nb), vanadium (V), chromium(Cr), manganese (Mn), cobalt (Co), lead (Zn), zirconium (Zr), molybdenum(Mo), Ta, tungsten (W), thallium (Tl), ruthenium (Ru), and iridium (Ir).

The catalyst material of the catalyst layer 14 of the electrode 11 asthe cathode is preferably Ag, Pd, Pt or the like. In addition, a metalcatalyst, a nitrogen-substituted carbon catalyst, an oxide catalyst,carbon and the like may be used in the catalyst layer 14.

When the electrode 12 is used for an anode of a fuel cell that performselectrolytic hydrogen generation, hydrogen oxidation/methanol oxidationand the like, a catalyst containing platinum such as Pt, PtCo, PtFe,PtNi, PtPd, PtIr, PtRu, and PtSn is preferably used for the catalystlayer 15.

When the electrode 11 as the cathode is used for the fuel cell or theoxygen reduction oxidation reaction of the oxygen-reducing element, thecatalyst containing platinum such as Pt, PtCo, PtFe, PtNi, PtPd, PtIr,PtRu, and PtSn is preferably used for the catalyst layer 14. Inaddition, the metal catalyst, a nitrogen-substituted carbon catalyst, anoxide catalyst and the like may be used for the catalyst layer 14.

The catalyst layer 15 of the electrode 12 as the anode is a catalystincluding an alternating catalyst layer structure (ACLS) catalyst havinga laminated structure of aggregate layers and void layers. The catalystlayer 15 includes the laminated structure, such that the durability andsurface area of the catalyst layer itself can be improved.

In the laminated structure of the catalyst layer 15, the plurality ofaggregate layers and void layers are alternately laminated. An averagethickness of the aggregate layer is 4 nm or more and 50 nm or less. Anaverage thickness of the void layer is 10 nm or more and 100 nm or less.

The catalyst layer 15 preferably has a porous structure including alarge number of voids therein. The shape of the void is not particularlylimited. Since there are voids inside the catalyst layer 15, a materialis smoothly transported, such that the water electrolysischaracteristics and the like are improved.

The laminated structure of the catalyst layer 15 is produced byalternately laminating a mixed layer of a catalyst material and apore-forming material and a pore-forming material layer by sputtering.After that, the pore-forming material contained in the mixed layer andthe pore-forming material layer are dissolved to be removed. By doingso, the catalyst layer 15 having the laminated structure in which theaggregate layer and the void layer are alternately laminated isproduced.

Since the inside of the catalyst layer 15 is partially bonded and thewhole catalyst layer is substantially constrained by the feeder, thelaminated structure of the catalyst layer is very stable against astress applied at the time of press or gas generation.

The catalyst layer 15 of the electrode 12 as the anode preferably hasthe laminated structure, and the method for producing the catalyst layer14 of the other electrode 11 as the cathode is not particularly limited.The catalyst layer 14 of the electrode 11 as the cathode, not includingthe laminated structure, may be produced on the feeder layer 16 by asputtering and may be produced by directly coating a suspension, inwhich the catalyst powder is dispersed with water, alcohol and the like,on the feeder layer 16. In addition, a coating method such aselectrolytic plating or thermal decomposition coating may also be used.It should be noted that when the feeder layer 16 itself serves as acatalyst, since the feeder layer 16 itself can form the cathode, thecatalyst layer is not required for the electrode. An example of thematerial forming the feeder may include a platinum-based precious metalcatalyst such as Pt.

<Membrane Electrode Assembly (MEA)>

The MEA 10 includes the pair of electrodes 11 and 12 disposed facingeach other and the electrolyte membrane 13 disposed between theelectrodes 11 and 12.

Here, FIG. 2 is an enlarged view of a cross section of the feeder layer17 of the electrode 12 as the anode.

An upper portion of FIG. 2 is a portion where the electrolyte membrane13 of the feeder layer 17 exists. The feeder layer 17 has a conductivematerial 50 and a void portion existing between the plurality ofconductive materials 50. The catalyst layer 15 is formed on a surface ofthe conductive material 50. From the upper portion of FIG. 2, theelectrolyte exists inside the feeder layer 17.

When observing the cross section of the feeder layer 17, it ispreferable that the electrolyte exists in a first region (D2) which isless than or equal to 80% of the thickness (D1) of the feeder layer 17,from the electrolyte membrane 13 toward the opposite direction to theelectrolyte membrane 13. In addition, it is preferable that the catalystlayer 15 exists in 50% or more of an outer circumference of the crosssection of the conductive material 50 in the first region.

In addition, here, a region other than the first region is defined as asecond region (D3). It is preferable that the catalyst layer 15 existsin 10% or less of the outer circumference of the cross section of theconductive material 50 in the second region.

Since the electrolyte exists inside the feeder layer within theabove-mentioned range, the electrolyte and the conductive material 50adequately contact each other, and the electrolyte is fixed to theinside of the feeder layer 17. In addition, since the electrolyte existsinside the feeder layer 17 within the above-mentioned range, a contactinterface between the catalyst layer 15 and the electrolyte is increasedand the reaction area is increased, such that a contact resistance canbe reduced. In addition, for example, in the electrolysis reaction ofwater, the electrolyte exists in the first region which is less than orequal to 80% of the thickness of the feeder layer 17, and therefore theoxygen generated by the oxidation reaction in the electrode 12 as theanode is discharged from the inside of the feeder layer 17 withoutinterfering with the electrolyte. Therefore, even when the ACLS catalystis used for the catalyst layer 15, it is possible to suppress theovervoltage of the MEA 10 from increasing.

Here, in order to confirm the electrolyte or the catalyst layer 15existing inside the feeder layer 17, the cross section of the feederlayer 17 is observed by scanning electron microscopy (SEM), and energydispersive X-ray spectroscopy (EDX) mapping or line analysis isperformed.

The surface of the cross section of the conductive material 50 is aposition in which the main constituent element of the conductivematerial 50 becomes 5% or less of all the detection elements. A surfaceregion of the cross section of the conductive material 50 is a regionfrom the position in which the main constituent element of theconductive material 50 becomes 5% or less of all the detection elementsto a position in which the count number of the main constituent elementof the catalyst layer becomes 1/10 as compared with its maximum point,in a direction away from the conductive material 50. However, thesurface region of the cross section of the conductive material 50 is atmost 5 um from the position in which the main constituent element of theconductive material 50 becomes 5% or less of all the direction elements,toward outside of the cross section of the conductive material 50.

A ratio (X) of an outer circumference where the catalyst layer 15 existsin the outer circumference of the cross section of the conductivematerial 50 in the first region (D2) in which the electrolyte existsinside the feeder layer 17, can be obtained by dividing the outercircumference (Xa) where the catalyst layer 15 exists in the surface ofthe cross section of the conductive material in the first region by thesum of the outer circumference (Xb) where the catalyst layer 15 does notexist in the surface of the cross section of the conductive material 50in the first region and the outer circumference (Xa) where the catalystlayer 15 exists in the surface of the cross section of the conductivematerial 50 in the first region. (X=Xa/(Xa+Xb)) The ratio (X) isreferred as the existence ratio (%) of catalyst layer on surface ofcross section of conductive material in region in which electrolyteexists.

The occupancy (existence ratio) (Y) of the electrolyte inside the feederlayer 17 is obtained by the EDX mapping. In the region where theconductive material 50 in the feeder layer 17 is removed, the region inwhich an existence ratio of fluorine or sulfur is 1/10 or less of theelectrolyte is calculated by the EDX mapping, which is referred to asthe region in which the electrolyte exists (Ya). The occupancy (Y) ofthe electrolyte inside the feeder layer 17 is obtained by, in a crosssection, dividing the area of the region in which the electrolyte exists(Ya) by the sum of the area of the second region in which the conductivematerial 50 in the feeder layer 17 is removed (Yb) and the area of theregion in which the electrolyte exists (Ya). (Y=Ya/(Ya+Yb)) The regionwhere the electrolyte exists is the first region, and the region otherthan the first region (the region where the electrolyte does not exist)is the second region. The occupancy (Y) is referred as the existenceratio (%) of electrolyte inside feeder layer.

In addition, the existence ratio (Z) of the catalyst layer 15 in thesecond region other than the first region inside the feeder layer 17 isa ratio of a region where the catalyst layer 15 exists in the surface ofthe cross section of the conductive material 50 existing in the regionin which the electrolyte does not exist. Specifically, it is a valueobtained by, in the second region (D3), dividing a sum of the outercircumferences of the portions where the catalyst layer 15 exists on thesurface of the cross section of the conductive material 50 (Za) by thesum of the outer circumferences of the conductive material 50 of thesecond region (Zb). (Z=Za/Zb) The ratio (Z) is referred as the existenceratio (%) of catalyst layer on surface of conductive material in regionin which electrolyte does not exist. The values X, Y, Z are averagevalues, respectively.

In the case of the actual evaluation, as viewed from the upper surfaceof the feeder layer 17, positions which are 10%, 25%, 40%, and 50% ofthe length of the feeder layer 17 are cut, and four cross sections areobserved by the SEM. The conductive material 50, the catalyst layer 15,and the electrolyte are confirmed and evaluated by the EDX mapping. Theabove-described parameters are calculated by averaging the valuesevaluated by sampling the four cross sections.

It should be noted that examples of a processing method for producing across section sample of the feeder layer may be milling by argon ions,or microtome processing after resin embedding and the like, but inconsideration of the processing damage, it is important that theprocessed feeder layer becomes close to the state immediately afterbeing pressed without being peeled off from the electrolyte.

<Electrochemical Cell>

Hereinafter, the electrochemical cell will be described.

The electrochemical cell (unit cell) 30 illustrated in FIG. 3 has thestructure in which the MEA 10 illustrated in FIG. 1 is sandwichedbetween an anode separator 32 and a cathode separator 31, respectively.

The anode separator 32 and the cathode separator 31 each includechannels 31 a and 32 a for supplying a reactant and a product to the MEA10. Seal members 33 and 34 are disposed on both side surfaces of thecatalyst layers 14 and 15 and the feeder layers 16 and 17, respectively,to prevent a fluid from being leaked from the MEA 10.

The plurality of unit cells 30 are laminated and connected in series toobtain a water electrolysis stack. The shape of the water electrolysisstack is not particularly limited, and is appropriately selectedaccording to a desired voltage or reaction amount. The fuel cell mayhave a flat disposition structure without being limited to a stackstructure. In addition, the number of unit cells included in the fuelcell is also not particularly limited.

As the reactant, for example, an aqueous solution containing at leastone selected from the group consisting of water, hydrogen, reformed gas,methanol, ethanol, and formic acid can be used.

The electrochemical cell according to the present embodiment may also bean electrolytic cell or a micro electro mechanical systems (MEMS) typeelectrochemical cell. For example, the electrolysis cell may have thesame configuration as the above-mentioned fuel cell except that theelectrolysis cell includes an oxygen generating catalyst electrode asthe electrode 11 instead of the anode.

Since the unit cell 30 includes the MEA 10, it is possible to suppressthe deterioration in the performance of the unit cell such as a cellvoltage for the reasons described above.

<Electrochemical Device>

Hereinafter, an electrochemical device 40 will be described.

The electrochemical device 40 is, for example, a hydrogen productiondevice.

FIG. 4 illustrates the electrochemical device 40 including theelectrochemical cell (unit cell) 30. The electrochemical device 40further includes a voltage applying unit (power supply) 43, a voltagemeasuring unit 41, a current measuring unit 42 and a controller 45. Bothelectrodes of the power supply 43 are electrically connected to theelectrode 12 as the anode and the electrode 11 as the cathode.

The controller 45 controls the power supply 43 and applies a voltage tothe electrochemical cell 30.

The voltage measuring unit 41 is electrically connected to the electrode12 as the anode and the electrode 11 as the cathode and measures thevoltage applied to the electrochemical cell 30. The measurementinformation is supplied to the controller 45.

The current measuring unit 42 is inserted into a voltage applicationcircuit for the electrochemical cell 30 and measures a current flowingin the electrochemical cell 30. The measurement information is suppliedto the controller 45.

The controller 45 has a memory. In accordance with a program stored inthe memory, the controller 45 controls an output of the power supply 43depending on each measurement information and performs a control toapply a voltage to the electrochemical cell 30 or change a load, and thelike.

It should be noted that when the electrochemical cell 30 is used for thebattery reaction, a voltage is applied to the electrochemical cell 30.When the electrochemical cell 30 is used for a reaction other than thebattery reaction, for example, the generation of hydrogen by waterelectrolysis and the like, a voltage is applied to the electrochemicalcell 30.

The electrochemical device 40 applies a voltage between the electrode 12as the anode and the electrode 11 as the cathode to progress theelectrochemical reaction.

Since the electrochemical device 40 includes the electrochemical cell30, it is possible to suppress the overvoltage together with reducingthe usage amount of the catalyst.

Hereinafter, the operation of the electrochemical cell 30 included inthe electrochemical device 40 will be described.

In the case of performing the electrolysis of water, if a voltage isapplied from the outside in the electrode 12 as the anode, water iselectrolyzed and reacts as the following Formula (1).

2H₂O→O₂+4H⁺+4e ⁻  (1)

Protons (H⁺) generated at this time pass through the electrolytemembrane 13, and electrons (e⁻) reach the electrode 11 as the cathodethrough an external circuit 44.

In the electrode 11 as the cathode, hydrogen is generated by thefollowing Formula (2).

2H⁺+2e ⁻→H₂  (2)

It is possible to produce hydrogen and oxygen by the above Formulas (1)and (2).

First Embodiment

The evaluation of the water electrolysis characteristics is performedusing the electrochemical device 40.

<Production of Electrode as Cathode>

705 mg of Pt/C (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd) ismixed with 5 cc of water and 3 mL of 5 wt % solution of Nafion(registered trademark) (manufactured by DuPont Co.). The mixed solutionis dispersed by an ultrasonic wave for 30 minutes to produce asuspension. The suspension is sprayed on water-repellent-treated (20 wt%) carbon paper (GDL 25 BC manufactured by CETEK Co., Ltd, thickness of0.32 mm, area of 235 cm²) and dried. The dried carbon paper is cut in 5cm×5 cm, which becomes the electrode 11 as the cathode.

<Production of Electrode as Anode>

A titanium non-woven fabric (fiber diameter of 30 μm) having anappropriate thickness and an opening ratio of 40% or a titanium particlesintered body (particle size of 100 μm) having an opening ratio of 85%is used as a base material with 5 cm×5 cm.

The pressure in the chamber is set to be 1 Pa, and in argon containing10% of oxygen, Ir and Ni are sputtered on the feeder layer 17 by asputtering method to form the catalyst layer 15.

The catalyst layer 15 includes IrO₂ as the catalyst aggregate layer anda void layer. The void layer is produced by RF sputtering of Ni at 500 Wfor 1000 seconds. After that, the catalyst aggregate layer is producedby RF sputtering of Ni at 200 W and Ir at 200 W for 100 seconds. Thesputtering of the void layer and the catalyst aggregate layer isrepeated 40 times. Thereafter, they are washed with nitric acid of 3Mand pure water to obtain the electrode 12 as the anode. The nitric aciddissolves Ni in the void layer and Ni in the catalyst aggregate layer.

<Production of MEA>

The MEA 10 is produced by hot-pressing the electrolyte membrane 13sandwiched between the anode electrode 12 and the cathode electrode 11at a temperature of 100° C. or higher and 230° C. or lower and at 5kg/cm² or more and 200 kg/cm² or less. The electrolyte membrane 13 issoftened by being hot-pressed at a temperature higher than or equal tothe softening point of the electrolyte membrane 13 and penetrates intothe inside of the feeder layers 16 and 17 in the electrodes 11 and 12.

Here, for example, Nafion (trademark, manufactured by DuPont Co.) havinga thickness of 127 μm, which is the electrolyte membrane 13, issandwiched between the electrode 12 as the anode and the electrode 11 asthe cathode. The Nafion is hot-pressed at a pressure of 1 ton at 120° C.to 230° C. for 3 minutes to produce the MEA 10.

<Production of Electrochemical Cell>

The MEA 10 is sandwiched between the anode separators 32 and the cathodeseparator 31, respectively. In addition, the electrochemical cell 30 wasproduced by disposing the seal members 33 and 34 on both side surfacesof the catalyst layers 14 and 15 and the feeder layers 16 and 17,respectively.

<Production of Electrochemical Device>

The plurality of electrochemical cells 30 were stacked and connected inseries to manufacture the electrochemical device 40.

<Evaluation of Water Electrolysis Characteristics>

The electrochemical device 40 is operated by applying a voltage betweenthe electrodes of the electrochemical cell 30, that is, between theelectrode 12 as the anode and the electrode 11 as the cathode by thepower supply 43. By doing so, water is electrolyzed to generate oxygenfrom the electrode 12 as the anode and generate hydrogen from theelectrode 11 as the cathode. At this time, a current density is 2 A/cm²,and an operation temperature is 80° C.

The relationship of the thickness of the feeder layer of the electrode12 as the anode, the shapes of the conductive material 50 and thecatalyst layer 15, the existence ratio (Y) (%) of the electrolyte insidethe feeder layer 17, the existence ratio (X) (%) of the catalyst layer15 on the surface of the cross section of the conductive material 50 inthe region in which the electrolyte exists, the existence ratio (Z) (%)of the catalyst layer 15 on the surface of the cross section of theconductive material 50 in the region in which the electrolyte does notexist, the cell voltage (V), and the performance determination, whichwere confirmed by the SEM-EDX mapping, is shown in the following Table1.

Here, the performance determination is indicated by “◯” when the cellvoltage is less than 1.88 V and is indicated by “x” when the cellvoltage is higher than or equal to 1.88 V.

It can be seen from the following Table 1 that (a) in the cross sectionof the feeder layer 17, the electrolyte exists in the first region of80% or less of the inside of the feeder layer 17, (b) in the firstregion, 50% or more of the catalyst layer 15 exists on the surface ofthe cross section of the conductive material 50, and (c) in the secondregion other than the first region, when the existence ratio of thecatalyst layer 15 on the surface of the conductive material 50 is lessthan or equal to 10%, the cell voltage becomes less than 1.88 V.

Generally, if the electrolyte penetrates into the inside of the feederlayer 17, the contact interface between the electrolyte and the surfaceof the cross section of the conductive material 50 is increased and thecontact resistance is decreased. Since the reaction proceeds at thecontact point between the electrolyte and the catalyst layer 15, it isnecessary to increase the contact interface between the catalyst layer15 and the electrolyte as much as possible. If the electrolyteexcessively penetrates into the conductive material 50, since theelectrolyte membrane 13 prevents the discharge of gas from the feederlayer 17, it is important how much the electrolyte penetrates into thefeeder layer 17.

Comparative Example 1

Table 2 shows the analysis results when the shapes of the conductivematerial and the catalyst layer are the laminated catalyst layer and thesingle-layered catalyst layer.

It can be seen from the following Table 2 that in both of the laminatedcatalyst layer and the single-layered contact layer, if each of theexistence ratio (%) of the electrolyte inside the feeder layer, theexistence ratio (%) of the catalyst layer on the surface of the crosssection of the conductive material in the region in which theelectrolyte exists, and the existence ratio (%) of the catalyst layer onthe surface of the cross section of the conductive material in theregion in which the electrolyte does not exist deviates from theabove-mentioned range, the performance deteriorates.

On the other hand, as the absolute value of the cell voltage, thelaminated catalyst layer is lower than the single-layered catalyst layerby 100 mV or more. It can be seen that the cell voltage can be loweredby forming the laminated catalyst layer.

Comparative Example 2

Table 3 shows the analysis results when the shapes of the conductivematerial and the catalyst layer are the cases whose catalyst layer 15 isa plated catalyst layer and a coated catalyst layer.

It can be seen from the following Table 3 that the condition that in thecross section of the feeder layer, the electrolyte exists 80% or less ofthe inside of the feeder layer 17, in the region in which theelectrolyte exists, the catalyst layer 15 exists on 50% or more of thesurface of the conductive material 50, and in the region in which theelectrolyte does not exist, the catalyst layer 15 exists on 10% or lessof the surface of the conductive material 50 is not satisfied.

Besides, the shapes of conductive material and catalyst layer are,“Non-woven fabric-Laminated catalyst layer” and “Particle sinteredbody-Laminated catalyst layer” in Table 1, “Non-woven fabric-Laminatedcatalyst layer”, “Particle sintered body-Laminated catalyst layer” and“Non-woven fabric-Single-layered catalyst layer” in Table 2, and“Non-woven fabric-Plated catalyst layer”, “Non-woven fabric-Coatedcatalyst layer” and “particle sintered body-Coated catalyst layer” inTable 3.

TABLE 1 Existence ratio (%) of Existence ratio (%) of Thickness catalystlayer on surface of catalyst layer on surface (μm) Shapes of conductiveExistence ratio (%) cross section of conductive of conductive materialin Cell of feeder material and of electrolyte inside material in regionin which region in which electrolyte voltage Performance layer catalystlayer feeder layer electrolyte exists does not exist (V) determination2000 Non-woven 1 100 1 1.85 ◯ 2000 fabric-Laminated 2 100 0 1.83 ◯ 200catalyst layer 20 100 2 1.84 ◯ 200 30 80 0 1.80 ◯ 50 75 100 10 1.84 ◯ 5080 100 0 1.81 ◯ 2000 Particle sintered 1 100 0.5 1.85 ◯ 2000body-Laminated 2 80 0 1.83 ◯ 200 catalyst layer 10 100 6 1.85 ◯ 200 2080 0 1.84 ◯ 200 30 51 0 1.83 ◯ 50 50 100 10 1.86 ◯ 50 70 87 0 1.84 ◯

TABLE 2 Existence ratio (%) of Existence ratio (%) of Thickness catalystlayer on surface of catalyst layer on surface (μm) Shapes of conductiveExistence ratio (%) cross section of conductive of conducive material inCell of feeder material and of electrolyte inside material in region inwhich region in which electrolyte voltage Performance layer catalystlayer feeder layer electrolyte exists does not exist (V) determination2000 Non-woven 5 40 0 1.99 X 2000 fabric-Laminated 6 33 0 2.00 X 2000catalyst layer 7 26 0 2.10 X 200 10 100 11 1.90 X 200 40 67 0 1.89 X 20050 40 0 1.91 X 200 60 33 0 1.96 X 200 70 28 0 2.00 X 50 10 100 80 2.10 X50 20 100 74 2.00 X 50 30 100 70 1.95 X 50 40 100 66 1.93 X 50 50 100 601.91 X 50 60 100 50 1.89 X 50 90 88 0 1.89 X 2000 Particle sintered 3 500 1.90 X 2000 body-Laminated 4 37 0 1.93 X 2000 catalyst layer 5 30 02.00 X 2000 6 25 0 2.05 X 2000 7 21 0 2.10 X 200 40 34 0 1.89 X 50 10100 55 2.00 X 50 20 100 50 1.93 X 50 30 100 42 1.89 X 50 90 70 0 1.89 X200 10 100 100 — — 200 30 100 100 — — 200 50 100 100 — — 200 70 100 100— — 200 Non-woven 10 100 11 2.20 X 200 fabric-Single-layered 20 100 21.98 — 200 catalyst layer 30 80 0 1.93 — 200 40 67 0 2.00 — 200 50 40 02.05 — 200 60 33 0 2.08 — 200 70 28 0 2.10 —

TABLE 3 Existence ratio (%) of Existence ratio (%) of Thickness catalystlayer on surface of catalyst layer on surface (μm) Shapes of conductiveExistence ratio (%) cross section of conductive of conductive materialin Cell of feeder material and of electrolyte inside material in regionin which region in which electrolyte voltage Performance layer catalystlayer feeder layer electrolyte exists does not exist (V) determination200 Non-woven 10 100 100 — — 200 fabric-Plated 30 100 100 — — 200catalyst layer 50 100 100 — — 200 70 100 100 — — 200 Non-woven 10 47 0 —— 200 fabric-Coated 30 41 0 — — 200 catalyst layer 50 32 0 — — 200 70 250 — — 200 Particle sintered 10 50 0 — — 200 body-Coated 30 45 0 — — 200catalyst layer 50 43 0 — — 200 70 38 0 — —

According to the embodiments, a membrane electrode assembly, anelectrochemical cell, and an electrochemical device capable ofsuppressing an increase in overvoltage is provided.

The existence ratio (Y) of the electrolyte in the feeder layer 17 ispreferable to be less than or equal to 80%. Although there is not alower limit, it may be 1% or more, or 4% or more. That is, theelectrolyte is preferable to be less than or equal to 80% of thethickness of the feeder layer, and though there is not a lower limit, itmay be 1% or more, or 4% or more. The existence ratio (X) of thecatalyst layer 15 on the surface of the cross section of the conductivematerial 50 in the region in which the catalyst exists is preferable tobe 50% or more. If desired, it may be a value of more than 50%, or morethan 70%. The existence ratio (Z) of the catalyst layer 15 on thesurface of the cross section of the conductive material 50 in the regionin which the catalyst is not present is preferable to be 10% or less.

Further, in the embodiments, the catalyst layer may be a laminatedstructure including an aggregate layer and a void layer. The thicknessof the aggregate layer may be 4 nm or more and 30 nm or less. A porosityof the feeder layer may be 40% or more and 90% or less. The feeder layermay be made of a metal non-woven fabric having a fiber length of 1 μm ormore and 100 μm or less. The feeder layer may be made of a metalsintered body of particles of 1 μm or more and 100 μm or less. Thethickness of the feeder layer may be 50 μm or more and 2000 μm or less.

While several embodiments of the present invention have been described,these embodiments have been presented by way of example and are notintended to limit the scope of the invention. These embodiments can bepracticed in other various forms, and various omissions, substitutions,and changes can be made without departing from the gist of theinvention. These embodiments or modified examples thereof are includedin the scope or gist of the invention as well as the invention describedin the claims and the equivalent scope thereof.

What is claimed is:
 1. A membrane electrode assembly, comprising: a pairof electrodes, each having a feeder layer that is porous and made of aconductive material; and an electrolyte membrane disposed between thepair of electrodes, wherein at least one of the electrodes has acatalyst layer disposed in the feeder layer, and, in a cross section ofthe feeder layer, an electrolyte exists in a first region less than orequal to 80% of a thickness of the feeder layer from the electrolytemembrane toward an opposite direction to the electrolyte membrane, thecatalyst layer exists at 50% or more of an outer circumference of across section of the conductive material in the first region, and thecatalyst layer exists at 10% or less of the outer circumference of thecross section of the conductive material in a second region other thanthe first region.
 2. The membrane electrode assembly according to claim1, wherein the catalyst layer is a laminated structure including anaggregate layer and a void layer.
 3. The membrane electrode assemblyaccording to claim 2, wherein a thickness of the aggregate layer is 4 nmor more and 30 nm or less.
 4. The membrane electrode assembly accordingto claim 1, wherein a porosity of the feeder layer is 40% or more and90% or less.
 5. The membrane electrode assembly according to claim 1,wherein the feeder layer is made of a metal non-woven fabric having afiber length of 1 μm or more and 100 μm or less.
 6. The membraneelectrode assembly according to claim 1, wherein the feeder layer ismade of a metal sintered body of particles of 1 μm or more and 100 μm orless.
 7. The membrane electrode assembly according to claim 1, whereinthe thickness of the feeder layer is 50 μm or more and 2000 μm or less.8. An electrochemical cell, comprising: a membrane electrode assemblyaccording to claim 1; and a pair of separators having the membraneelectrode assembly sandwiched therebetween.
 9. An electrochemical devicecomprising the electrochemical cell according to claim
 8. 10. Theelectrochemical device of claim 9, wherein the electrochemical device isa hydrogen production device.